1. Field of the Invention
The invention relates to a force sensor that converts forces to electrical information such as capacitance and the like, in particular, to a force sensor, a force detection system and a force detection program used for conversion of manual inputs and the like into electrical signals.
2. Description of Related Art
The sensor for converting a force to capacitance is typically used for a bathroom scale and the like. This type of force sensor typically has an operation input unit constituted as a button type or a stick type to serve as a man-machine interface and is used, for example, as a stick type input device of a notebook type personal computer.
FIG. 1 and FIG. 2 show the outline of a general force sensor of the prior art, wherein FIG. 1(A) shows its plan view, FIG. 1(B) shows its IB-IB cross section, FIG. 1(C) shows its detection electrode, FIG. 2(A) shows its plan view, FIG. 2(B) shows its IIB-IIB cross section, and FIG. 2(C) shows its detection electrode. A force sensor 2 has a structure 6 consisting of insulators such as silicon rubber and conductors provided on a substrate 4, as well as an electrode 8 on the ceiling of the structure 6 and a detection electrode 10 facing the electrode 8 on the side of the substrate 4. If the electrode is made of the conductor of the structure 6, it is not necessary to provide the electrode 8 independently. The difference here is that the device shown in FIG. 1 has a structure 6 formed as a beam supported on both ends and a circular detection electrode 10, while the device shown in FIG. 2 has a structure 6 formed as a cantilever beam and a rectangular detection electrode 10.
The description of such a force sensor 2 can be tried using the force sensor 2 shown in FIG. 1 as an example, wherein a force “f” applied on the structure 6 causes the structure 6 to deform as shown in FIG. 3(A), which in turn reduces the electrode distance “d” and eventually causes the electrode 8 to contact the detection electrode 10 as shown in FIG. 3(B), increasing the capacitance between the electrodes “C.” The relation between the input (force “f”) of the structure 6 and the output (capacitance “C”) varies in a smooth curve as shown in FIG. 4. COffset is the capacitance between the electrodes in the condition shown in FIG. 1(B), i.e., the offset output when the electrode distance “d” has not changed, and this offset output indicates that the capacitance does not change with the force “f” unless the force “f” does not exceed the elasticity limit of the structure 6, i.e., it represents the zero point output. This offset output depends on the elasticity, restoring capability, permanent strain, etc., of the structure 6. The same input/output relation exists for the force sensor 2 shown in FIG. 2.
A sensor that provides the change of capacitance C as an output relative to the force “f” applied is disclosed by JP-A-6-314163.
A capacitance type sensor disclosed in said document has a substantial displacement of the input part, and is constituted with two substantial substrates provided in parallel positions to allow them to move in parallel to each other, where electrodes are provided on their opposing surfaces respectively and positioned at 90° angles to each other.
The reliability of the input-output relation is critical in case of such a force sensor, in particular, the stability of said zero point output (offset output) is extremely important. Therefore, it is necessary for the output to be zero or present a specific offset value and the value is constant when it is not operated. In other words, the reliability of the detection device will be lost if the output varies or the offset value varies when it is not operated, making it impossible to determine the zero point.
For example, a pointing device using a force sensor may cause such a problem that a minute output signal causes to move the pointer when the zero point or the offset value changes when there is no operation. For example, a case has been reported that a pointer started to move by merely receiving air from an air conditioner unit entirely unrelated to the user's intention.
The input/output relation of the force sensor requires that there is a certain relation between input and output, it is extremely difficult to distinguish the output drift due to the environmental change and the output change due to a minute input when a minute force is to be detected under a condition where there are output drift due to changes in environmental factors such as temperature. Trying to stabilize the output against changes in environmental factors such as temperature tends to be expensive.
Certain variances are typically expected for a force sensor's zero point output (offset output), which varies with each force sensor, so that it is meaningless to try to set the output value uniformly. Therefore, what is commonly done is to store the value when the power source voltage is turned on as the zero point value forcibly in a memory or to store the zero point value (offset voltage value, etc.) in an involatile memory at the time of adjustment prior to the factory shipment for each force sensor. A problem with forcibly storing the value when the power is turned on as the zero point value is that a value different from the true value can be recorded as the zero value in such a case as when the operator's finger is touching the unit by mistake. A problem with the use of an involatile memory device for storing the zero point value at the time of a factory shipment is that it means an additional cost. In any case, the zero point value at the time of storage can be affected by changes of environmental factors such as temperature and humidity, so that it is impossible to follow such zero point fluctuations.
Although it is effective in saving power in a system using a force sensor to avoid powering the unit when there is no input to the power sensor, it makes it impossible to identify if there is an input or not unless the detection unit is powered. In order to avoid such an inconvenience, it is a common practice to power the unit intermittently.
Let us describe the method of zero point setting of the force sensor (FIG. 1 or FIG. 2) referring to FIG. 5. In FIG. 5(A), five inputs f1, f2, f3, f4, and f5 are entered within a short spun of time, wherein the size relation between the inputs f1 through f5 is f2>f1>f3≈f4≈f5, i.e., f2 has the highest level, while f1 and f2 have the longest time span, and f3, f4 and f5 are minute inputs of minute time spans. Moreover, it is assumed that minute inputs are entered after a gigantic input.
For such inputs f1 through f5, a force sensor 2 generates outputs C11, C12, C13, C14 and C15 in correspondence with the inputs as shown in FIG. 5(B), i.e., showing changes of capacitance in correspondence with the input, a minute output is generated in the b1 section after the output C11 for the first input f1 despite the fact that there is no input. This is the residual output. Moreover, a large residual output is generated in the b2 section immediately after the output C12 due to the input f2. Since the outputs C13 through C15 are overlapping on said residual output and the inputs are smaller after b3, it is understandable that the residual output reduces with time.
These residual outputs are dependant on the restoration capability of the structure 6 that receives pressures. Elastomer such as rubber used for the structure 6 has a property not to restore to its original shape after a deformation. The restoration property can be improved to a degree with a proper selection of material to reduce residual outputs but cannot eliminate them completely. When strong input changes occur and their time intervals are shorter, it is difficult to eliminate residual outputs.
A method of eliminating such residual outputs has been proposed in which a dead zone region for not allowing the residual outputs to be responded is set up near the zero point and forcing the output range of the dead zone region to be assigned to the zero point as shown in FIG. 5(C). FIG. 5(D) shows the outputs C110, C120, C130, C140, (C150) that have levels lowered as a result of compensations due to the dead zone region setup. When such a dead zone region is set up, it eliminates the problem of residual outputs but it affects the input/output relation at the same time. In a comparison between the output (B) where the dead zone region is not set up, and the output (D) where the dead zone region is set up, the delay time td will be generated between the input and the output, thus deteriorating the time-related response. Moreover, the minute output C15 can become buried into the dead zone level and may not generate any output (“d” section), thus deteriorating the level-related response. The forced zero point setup according to the dead zone region setup may deteriorate the detection sensitivity and the input relation and affect the reliability of the force sensor.
There is no mention of such a problem and no disclosure or suggestion is made as a means of solution for it in the above-mentioned prior art document.